The present invention relates to a pneumatic tire, more particularly to a tread pattern suitable for studless tires comprising four circumferential rows of tread blocks capable of improving the noise performance.
In general, a studless tire is provided in the tread portion with a large number of sipes to improve running performance on icy roads. Further, the tread portion is provided with tread grooves (axial grooves and circumferential grooves). Such tread grooves are formed relatively wide in comparison with those of summer tires in order that when running on a snowy road a relatively large volume of snow can be packed into the tread grooves existing in the ground contacting patch because a relatively large volume of packed snow in the tread grooves provides a relatively large shearing force (herein after “in-groove snow shearing force”), therefore, road grip performance (braking force, traction force) on snowy roads is improved.
In recent years, on the other hand, with improvements in the tread rubber compounds of studless tires, there are increasing opportunities for the studless tires to run on dry asphalt roads. As a result, the studless tires are also required to reduce their noise emission equally to summer tires.
In the case of studless tires, the tread portion is relatively soft due to the tread compound and a large number of sipes, as a result, it is more effectual for reducing the noise emission to reduce the sound pressure level of so called air resonance noise. The air resonance noise is a noise sound caused by resonance of air in a tube like part formed between a wide straight circumferential groove in the ground contacting patch and the road surface.
If the width of the circumferential groove is decreased, the air resonance therein can be suppressed, but deterioration of on-the-snow running performance and wet performance is inevitable.